Known high pressure nozzles for descaling steel products are constructed as flat-spray nozzles. The mouthpiece for such descaling nozzles conventionally has a discharge opening to which is connected a jet-forming discharge cone. European patent EP 792 692 B1 e.g. discloses a mouthpiece for a descaling nozzle, in which a discharge chamber tapering towards the discharge opening passes downstream of the latter into conically widening boundary surfaces of the mouthpiece. These boundary surfaces define the flat jet formed with respect to its lateral extension. The discharge opening and discharge cone can be elliptical.